Granularity controlled percolative current conduction in YBa2Cu3O7/Ag composite thick films

Temperature-dependent electrical resistivity in a set of YBa2Cu3O7/Ag thick films prepared by reaction diffusion technique is analysed to estimate the extent of granularity in them. Percolative nature of charge transfer in the films was examined by studying the excess conductivity in the superconducting order parameter fluctuation (SCOPF) region (T greater than or equal to T-c), the approach to zero-resistance state in the tail region, the current-controlled tailing of the resistivity transition near T-c0 (T-c0 < T < T-c0) and the current-voltage characteristics in the paracoherent regime (T < T-c0) with their associated exponents. The percolative current model was thus utilized to quantify granularity in the films of such parameters like the weak link resistivity across grain boundaries, current path lengthening factor arising due to grain misalignment, and due to voids and cracks. Evolution of these parameters with Ag content in the films indicates reduced structural defects (voids and cracks) at higher Ag concentrations. The result points to a mechanism of grain growth in the composites, which accounts for the improved grain growth with narrower grain size distribution in these systems as compared to that in Ag-free YBCO sintered material. These microstructural modifications cause lowering of percolation threshold from the value expected in a granular system where the grain size approaches to one of the system dimensions (the film thickness) making the conduction through grain boundary essentially two-dimensional.